Collagen is a main protein component of extracellular matrix filling the gap between cells or between cell populations of biological tissues, and is said to sometimes make up nearly 30% of the total protein of the body in mammals (see non-patent document 1).
Collagen forms fibrous or membrane structures, has a primary function of maintenance, support, connection, reinforcement and the like of tissue structures, and is present in the skin, bone, tendon, ligament, cornea, blood vessel and the like in large amounts. Decrease, denaturation and the like of tissue collagen are considered to be the major factors causing wrinkles, sagging, osteoporosis and the like, which are developed by aging of these tissues. In fact, reports have also documented that tissue collagen decrease strikingly by exposure to sun light for a long time (see non-patent document 2), and that age-related low metabolism of biogenic substances advances as triggered by low collagen metabolism (see non-patent document 3).
In addition, it is known that the weight ratio of alanine and hydroxyproline from among the amino acids constituting collagen or gelatin is 1:1.3 (see non-patent document 4).
As a method for enhancing collagen metabolism, a collagen synthesis promoting action by collagen degradation product is known (see patent documents 1-4).
In addition, hydroxyproline is known to have a collagen synthesis promoting action on cultured human fibroblasts, and a wrinkle formation suppressive or improving effect by topical application (see patent document 5).
Furthermore, amino acid-containing cosmetics which contain collagen-constituent amino acids such as glycine, proline, alanine and the like (see patent document 6), and such cosmetics containing hydroxyproline (see patent document 7) are known.
However, it is considered that the effect of collagen-containing foods or drinks is hardly felt unless it is ingested in the order of several grams, and also, the smell or taste in some ingestion form prevents easy ingestion. In view of such problems, a new method capable of decreasing the ingestion amount of collagen has been desired.
Moreover, a combination effect of collagen or gelatin or a degradation product thereof, and hydroxyproline is not known.    non-patent document 1: “Seikagaku Jiten (Biochemical Dictionary) first edition”, Tokyo Kagaku Dojin, 1984, p. 480    non-patent document 2: “Karei to Hihu (Aging and Skin)”, Seishi Shoin, 1986, p. 35    non-patent document 3: “Mechanism and Control of Aging”, IPC, 1993, p. 151    non-patent document 4: “Biochemical Data Book I”, The Japanese Biochemical Society, 1981, p. 1667    patent document 1: JP-A-7-278012    patent document 2: JP-A-2000-201649    patent document 3: JP-A-2001-131084    patent document 4: JP-A-2003-137807    patent document 5: JP-A-7-278012    patent document 6: JP-A-2001-224334    patent document 7: JP-A-2006-79921